The present invention relates to a ringing signal transmission system associated with a multi-channel access radiotelephone system, particularly a maritime mobile radiotelephone system having a base station (e.g. coast station) connecting to a public telephone swiching network, for transmitting a ringing signal which is generated when a subscriber of the switching network calls an end station (e.g. ship station) of the radiotelephone system.
In a radiotelephone system of the kind described, e.g., a maritime mobile radiotelephone system connected to a telephone exchange of a public telecommunication network, it has been customary to effect connection control and, then, transmission of a ringing signal in the following manner when an end station of the radiotelephone system is called by a subscriber of the telecommunication network. Upon receipt of a call from a subscriber of the telecommunication network, the telephone exchange is caused into connection with a line concentrator so as to send a ringing signal to a called subscriber's line by way of a base station connected to the line concentrator, i.e. coast station. The line concentrator recognizes the call by detecting the ringing signal and starts on a connection with a ship station corresponding to the subscriber's line. In this connection responsive to a call, while various signals are exchanged between the line concentrator and the ship station as well known in the art, the method of exchanging such signals is not directly relevant to the present invention, and therefore, details of the connection will not be described herein. Upon completion of the connection, a telephone associated with the ship station is caused to produce a tone. For producing the tone, the line concentrator sends a tone signal to the ship station in synchronism with a ringing signal which is sent thereto from the telephone exchange.
Generally, the ringing signal applied from the telephone exchange to the line concentrator is an intermittent signal which may be transmitted for one second and interrupted for the subsequent two seconds, for example. Therefore, when the line concentrator after detection of a one-second transmission time of the ringing signal has started on a connection and, then, the connection has been set up at the beginning of a two-second interruption time, the telephone associated with the ship station does not immediately produce a calling tone despite the completion of the connection. That is, the calling tone does not occur until the ringing signal reappears after the two seconds of interruption. Stated another way, the subscriber at the ship station cannot recognize the receipt of a call until the tone is produced from the telephone. It follows that when the subscriber off-hooks during the two-second interruption time of the ringing signal with the intention of originating a call, he or she suddently hears subscriber's voice from the telecommunication network on the land via the coast station. Such would raise strange and uneasy sensation in the subscriber at the ship station.